CA1050788A - Dental bridge alloy - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A dental alloy suited for crown and bridge applica-tions essentially free of carbon and molybdenum and having an alloy base including, by weight, 10 to 60 parts of cobalt, 17 to 24 parts chromium, 20 to 75 parts nickel as the essential major alloying elements, and a member of the group comprising tantalum and niobium alloyed therewith to promote uniformity and fineness of crystal size, said alloying element being present in the amount of about 2 percent to 6 percent tantalum or the equivalent atomic weight of niobium.

Description

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This invention relates to a dental alloy suited for the production of fixed dental restorations such as crowns and bridges. More particularly, -this invention relates to a nickel-cobalt base dental alloy which is essentially free of molybdenum~ tungsten, carbon and boron and has as its principal constituents cobalt, chromium and nickel, and contains small amounts of niobium or tantalum.
Although it has been stated in the past that alloys suitable for casting dentures are also suitable for casting fixed bridges and crowns, there are major differences in the mechanical properties required or these different appli-cations.
At present, Type III gold alloys are universally used for fixed crown and bridge prosthesis since these are the only currently available alloys which possess the mechani- -cal properties and have the melt-to-melt uniformity and appropriate crystal size and distribution for successful use in such dental applications~
An alloy for casting crowns and bridges must be burnishable which requires the low hardness associated with a low yield strength while one for casting dentures should be at least twice as strong and hence cannot be burnishable.
Burnishability may be defined as the ability of the metal to be worked in the mouth of a patient by small hand instruments, called burnishers, such that open margins of the alloy casting are closed and conform precisely to the cavity margins in the tooth. The act of burnishing consists of indenting the alloy with a burnisher by hand ~orce, bend-ing the margin of the casting to close the open margin at the metal-tooth interface, and stretching and working the 7~
metal to match the contours of the tooth at the cavlty mar-gins. To be able to indent the cast crown with a hand in-strument, the alloy must possess low hardness. To be able to bend the margin of the cast crown, the proportional limit of the metal alloy must be low enough so that it can be readily exceeded by the pressure of the hand and hence the alloy must possess low yield strength. To be able to mani-pUlàte the alloy and shape it to precise:Ly match the contours of the natural tooth, ~he alloy must be malleable. The worka-bility or malleability of the alloy is dependent not only ona low yield strength but also depends on high ductility.
Hence, an alloy for crown and bridge castings should possess low hardness, low yield strength and high ductility.
For use in partial denture applications, an alloy must have high yield strength. Without high yield strength, the clasp components of a partial denture will deform during insertion or removal or under the masticatory loads encoun-tered in use. A desirable denture alloy must have a yield strength of 70,000-80,000 psi while the most desirable crown and bridge alloy available is dental Type III gold alloys which have a yield strength of 27,000-34,000 psi. By virtue of the fact that the alloy possesses such low yield strength, its hardness is low enough to permit its indentation by manual pressure.
Also while in partial denture alloys 3 a maximum of 10 percent elongation is generally adequate to permit adjust-ment of the clasp components periodically, the ductility requirements of a crown and bridge alloy is much larger as exemplified by the fact that dental Type III gold alloys universally used for crown and bridge casting possesses a 5~7~8 ductility of 22-27 percent elongation.
Ef~or-ts to introduce cobalt-nickel-chromium alloys for the processing of removable dentures was prompted by their high strength, corrosion resistance and low cost n These alloys were characterized by low ductilities until 1967 ~hen Asgar's U.S.
patent 3,544,315 introduced such an alloy possessing a ductility of up to 10 percent elongation. The level of ductility required for crowns or bridges, where not only bending but burnishing is necessary, has been lacking in the cobalt-nickel-chromium alloys available for dental uses. Crown and bridge applications require a ductility of at least 20 percent elongation.
The present invention provides cobalt-niekel-ehromium alloys which have properties e~ual to or exeeeding those oE Type III gold alloys currently used for the construction of dental crowns and bridges.
The present invention also provides alloys with mechan- ;;
ical properties not controlled by the conventionally known ;`
strengthening mechanisms of solid solution hardening or preeipi-tation hardening but by controlling the crystal structure of the alloys through controlling the stacking fault energy of the alloy as well as the uniformity and fineness of the metal crystals.
The present invention further provides such an alloy which possesses casting properties equal to or better than Type III gold alloys and can be fabrica-ted by substantially the same current conventional dental proceduresO Included in this is the provision of such an alloy which is as ductile as Type III gold alloys currently used in crown and bridge applications.
The present invention also provides a cobalt~nickel-chromium alloy which possesses a yield strength of no more than 35,000 psi and a ductility of at least 20 percent elongation.
The present invention again provides a cobalt-nickel-chromium alloy wherein the ratio of eobalt to niekel and chromium is adjusted to be readily cast, Xree from blow holes and voids, .

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into sheets as thin as 2~ gauge and to which niobium and/or tantalum is added to stabilize its crystal structure in the desirable face cen~ered cubic crystal lattice structure as well as to act as a nucleating agent for the crystals of the alloy base.
According to the present invention 1here is provided a highly ductile cobalt-chromium-nickel dental alloy having a .
ductility of about 20 percent or more:elongation, a yield strength oE about 35,000 or less and low work hardening characteristics suited for crown and bridge applications requiring deformation :
by hand burnishing in the mou-th of a patient, said alloy having no more than 0.02 percent carbon and being essentially free of boron, molybdenum, titanium, aluminum and tungsten to pre~ent the formation of hardening precipitates thereof in the alloy and having an alloy base consisting essentially of, by weight, about lO to 60 percent cobalt, 17 to 24 percent chromium, and 20 to 75 percent nickel as the essential major alloying elements, and an .
element of the group consisting of tantalum and niobium alloyed therewith to promote a high rate of crystallization and provide -~.
uniformity and fineness of crystal size, said alloying element being present in an amount of up to about 4 percent tantalum or an equivalent atomic weight of niobium.
The present invention also provides the method of making a cobalt-nickel-chromium alloy suited for use as a gold alloy substitute for dental crown and bridge applications re- .`
quiring hand burnishing in the mollth of a patient and having a controlled high stacking fault energy, a ductility of about 20 percent ormore ~longation, a yield strength of a~out 35,000 psi .
or less, and low work hardening characteristics, said alloy having an alloy base consisting essentially of about 20 to 75 percent nickel, 17 to 24 percent chromiu~, and lO to 60 percent cobalt, by weight, comprising the steps of providing nickel in an amount of about twice as much as the chromium present, of 05~78~
providing the remainder of the alloy base by adding cobalt and an additional amount of nickel in the ratio of nickel to cobalt of between about 1:3 and 2:1 by weight and the step of stabilizing the uniformity and fineness of the crystal structure of the alloy by adding an element selected from -the group consisting of niobium and tantalum in an amount of up to about 4 percent tantalum, by weight, or the equivalent atomic weight of niobium to promote a high rate of crystallization and provide fineness and uniformity of grain size in the alloy.
The invention accordingly comprises the combination of elements disclosed herein and articles possessing the features, properties; and characteristics which are exemplified in the following disclosure.
When the element cobalt is heated about 500C, its crystal structure is the ductile face centered cubic latt.ice (FCC). Upon cooling below 417C, the crystal structure trans-forms to the less ductile hexagonal close packed structure (HCP).
This is due to the formation of faults in the stacking of atoms below 417dC and hence cobalt is said to have low stacking fault energy (SFE), and undergoes allotropic transformation upon cooling. This allotropic transformation of cobalt is not desir~
able for crown and bridge alloys because it markedly reduces ductility. The SFE of cobalt can be raised, and hence its allo-tropic transformation is inhibited, by the addition of certain .

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; elements.
The addition of more than 26 percent nickel to cobalt in a binary cobalt-nickel alloy under equilibrium conditions, will inhibit the allotropic transformation and maintain the ductile FCC structure at room temperature.
However, the addition of more than 65 percent nickel in the binary alloy system is harmful to the desirable pro-perties of low proportional limit and high ductility because concentrations higher than 65 percent nickel causes the formation of the ordered superlattice CoNi3 which raises the proportional limit and hardness and lowers the ductility.
Chromium, on the other hand, has the opposite effect of nickel on the SFE of a cobalt containing alloy, i.e., the addition of chrom;um lowers the SFE of cobalt resulting in the formation of more of khe less ductile HCP structure at room temperature. However, the addition of chromium is essential because it imparts mandatory corrosion resistance to the alloy. When chromium is added, the minimum nickel content must be increased. Two atomic percent of nickel added to the alloy to counteract the adverse efEect of one atomic percent of chromium on SFE. Since their atomic weights are almost equal, the addition of one percent o:E
chromium, by weight, must, in the preferred embodiment of this invention, be offset by increasing the nickel content of an alloy by 2 percent9 by weight, in order to maintain the same SFE in the alloy at room temperature.
Other corrosion inhibiting elements, such as molyb denum, have an adverse e-ffect on the alloys of this invention due to an unfavorable effect on SFE and the formation of hardening precipitates that reduce ductility. Accordingly, -5- `~

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~ 05a7~8 a minimum o-f about 20 percent chromium by weight, should be used in the alloys.
As explained above, the effect of this amount of chromium on SFE must be balanced by the addition of 40 per-cent nickel, by weight, so that 60 percent of the alloy composition is consumed by this essential balance of chro-mium and nickel.
The remaining 40 percent o-f the alloy should, in the preferred embodiment, be balanced in its ratio of cobalt to nickel in accordance with the equilibrium conditions dis-cussed above. Therefore, the minimum nickel content to produce an FCC binary cobalt-nickel alloy should preferably be 26 percent and the maximum should preferably be 65 per-cent. When the minimum nickel content necessary to maintain the FCC structure at room temperature is considered, the remaining 40 percent of a cobalt-nickel-chromium ternary alloy must be in the ratio of 26 percent nickel to 74 per-cent cobalt, or in the ratio of about 1 to 3. This would ~ r result in the incorporation of an additional 10 percent nickel and 30 percent cobalt to form the ternary alloys.The preferred ternary alloy composition which contains minimum nickel content to maintain the FCC structure at room temperature should, therefore, be 20 percent chromium, 50 percent nickel and 30 percent cobalt, by weight.
When maximum nickel conten~ is considered on the other hand, the maximum nickel content in the remaining 40 percent to form the preferred ternary alloy could be as much as 65 percent or in the ratio of 65 percent nickel to 35 percent cobalt, or in the ratio of about 2 to 1. In the latter case, the ternary alloy that contains maximum ~ 19~7~ :
nickel content and maintains the FCC structure at room tem-perature without the precipitation of ~oNi3 is Z0 percent chromium, 67 percent nickel and 13 percent cobalt, by weight.
As indicated in the tables below, alloys 5 and 7 which possess the compositions indicated above are charac-terized by mechanical properties better than those of gold Type III alloys for crown and bridge applications especially when burnishability is considered. In fact alloys 5, 6, 7 and 8, which according to the above considerations crystal-lize in the FCC structure 9 are of almost identical proper-ties indicating that variations in nickel content -from 50-67 percent and in cobalt content from 13-30 percent has no major influence on the mechanical properties so long as these ratios of these elements to each other as well as to chromium conform with the SFE considerations given above and the alloy is composed to crystallize in the desirable FCC structure.
An alloy for casting crowns and bridges should also possess good flowability in its molten state so that it can be cast into the continuous thin films which are necessary for the production of castings with the fine thin margins that are required by crown and bridge applications.
Thin castings are also necessary in producing what are termed veneer crowns where a cast crown which covers a -prepared tooth is provided with an extremely thin cross-section on the facial surface to receive a porcelain or acrylic overlay for cosmetic purposes. To meet the standard test of flowability necessary for crown and bridge applica-tions through the use of conventional dental procedures, an ~ L~s~
alloy must be capable of being consistently cast, free of blow holes and voids, into a sheet of metal 28 gauge thick~
3/8 inch wide and 1 3/4 inches long.
Currently available cobalt-nic~el-chromium dental al-loys having the highest flowability in the molten state, A l such as those disclosed by Asgar~patent 3,54~,315, Touceda ~patent 2,103,500, and Prosen~patent 2,674,571 may not be cast into sheets as thin as 28 gauge due to their inadequate flowability in the ~olten csndition. These alloys, which contain molybdenum to provide the strength properties re-quired, do not meet the standard test for -Elowability re-quired -for crown and bridge applications.
It is evident from Table II below that the mechanical properties of alloys 5-8 vary over a wide range of up to 35 percent from one melt to another for the same composition.
This variation is highly undesirable for commercial produc-tion and clinical manipulation and may be due to the fact that not only must an alloy have a high SFP, which was achieved through balancing the cobalt-nickel-chromium ratios in the alloy, but rapid crys~allization of the liquid alloy is also necessary to maintain the desirable FCC structure.
Raising SFE inhibits the transformation of the element co-balt to HCP upon cooling. ~owever, the inherent tendency in the metal to transform continues to remain and tends to take place when the solidification is slow. In order for the ternary cobalt-nickel-chromium molten alloy to solidify, nuclei of crystallization -first have to form. In the ab-sence of additional elements, only a few nuclei are formed and the crystals begin to grow slowly to form a casting consisting of a few large crystals. Since the growth of ~ os~
these crystals is slow due to the small number o~ nucleation sites, cobalt will have the opportunity to transform. When the alloy possesses high SFE but slow rate o-f crystalliza-tion, cobalt transforms partially and the quantity trans-formed is not controlled and varies from melt-to-melt dependent upon many environmental factors surrounding the liquid metal and thus major variations in the mechanical properties occur from one melt to another. Accordingly, in order to completely inhibit the FCC to ~CP transformation, not only is a high SFE necessary but a high rate of crystal- ~ -lization is also necessary.
A high rate of crystalli~ation of the cobalt-nickel-chromium system can be achieved by the addition of nucleat-ing agents. Any elements added to alloys meant for crown and bridge applications to provide nucleation sites, should be selected to also raise the SFE of the alloy. Further, they should possess an adequate solubility in, and a high melting point relative to, the base alloy to enable them to act as nucleating agents and should not deteriorate the mechanical properties of the alloy. Niobium and tantalum meet these requirements. As illustrated by examples 13-22 it was discovered that the addition of 2 percent tantalum or 1 percent niobium will accelerate the crystallization of the alloy through acting as nucleation sites due to their adequate solubility in the alloy and their high melting points of 2996C and 2415C; respectively. Niobium has a solubility of about 3 percent and tantalum of about 6 per-cent in the alloy. Alloys 13-22 are substantially of the same composition as alloys 4-8 except for their tantalum or niobium contant. It is evident from Table II that the ,. .~

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variations in the mechanical properties of alloys 13-22 were reduced to the acceptable 5-10 percent by the addition of niobium or tantalum to produce alloys of similar properties from one melt to another of the same composition.
~` When SFE considerations were not followed and cobalt-nickel-chromium ratios were reduced below the minimal nickel requirements as shown in examples 9-12, not only did the melt-to-melt variation in the mechanical properties reach almost 100 percent, but the property of ductility also de-' 10 teriorated to below 20 percent and further to 5 percent elongation.
Although alloys 9, 10 and 11 should contain higher concentrations of the HCP phase, their mechanical properties make them suitable for crown and bridge applications. Natur-ally, such use is contingent upon controlling the chemistry ', of the alloys such that the variation from melt-to-melt for -~
the same composition is reduced. The addition of 2 percent tantalum or 1 percent niobium to examples 9-11 reduced the extent of variation somewhat but not to the desired limits.
However, the use of 4 percent tantalum or 2 percent niobium ;j reduced the variation in the mechanical properties to within acceptable limits. In alloys 9-11 which have a high cobalt content, higher concentrations of tantalum and niobium were necessary because the alloys had lower SFE and larger quan-tities of tantalum or niobium were necessary to raise it.
Examples 23-28 are essentially alloys 9-11 after the addition of tantalum or niobium. Enough of the latter elements not only reduced variation from melt-to-melt, but also raised the yield strengths of the alloys and lowered their ductilities. The latter effect may be attributed to .

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the fact that, at these concentrations, both niobium and tantalum rendered significant solid solution hardening of the alloy and/or caused the precipitation of small amounts of intermetallic compounds.
Niobium and tantalum have the effect of counteract-ing the harmful effect of chromium on the SFE of a cobalt containing alloy.
Specifically, one atomic percent of niobium counter-acts the harmful effect of 2.5 atomic percent of chromium on the SFE. The atomic weight of chromium is roughly half that of niobium which means that the ef-fect o-f one percent of nio-bium, by weight, counteracts the effect of 1.25 percent of chromium, by weight. On the other hand, the addition of more than about 2 percent of niobium causes the precipitation o-f intermetallic compounds and deteriorates ductility. The ad-verse effect on SF~ of the 20 percent chromium required in a dental alloy, may be counteracted by the addition o-f both niobium and nickel. Since 2 percent niobium is a desirable maximum and counteracts the effect of 2.5 percent chromium, the effect of only 17 1/2 percent chromium must be counter-acted by nickel when 2 percent niobium is present. Since 2 percent nickel, by weight, counteracts the effect of 1 per-cent chromium, by weight, a minimum amount of 35 percent nickel, by weight, must be included in such a composition.
Naturally, a decrease in the concentration of niobium must be accompanied by an increase in ~he concentration of nickel.
For instance, decreasing the niobium content of the alloy by one percent must be accompanied by increasing nickel by

2.5 percent to maintain the same crystal structure and sub-stantially the same properties in the final alloy. Tantalum can be substituted for niobium in about the ratio of 2 to l, by weight, because it has an equivalent effect on SFE
to that of niobium but twice the atomic weight. One per-cent of niobium is ef-fective but a range of l to 3 percent can be used with the higher percentage of niobium being used for alloys with a low nic~el-cobal1 ratio In addition to the aforementioned essential elements, some other metallic and non-metallic elements may be present in the alloy, some as accidental impurities. Other elements are markedly detrimental to the mechanical properties re-quired of crown and bridge applications and their presence in siginificant amounts cannot be tolerated.
Carbon, boron, gold zirconium~ manganese, copper, aluminum, titanium, tin and iron have varying degrees of favorable effect on the SPE of a cobalt containing alloy.
Carbon and boron are highly effective in raising the SFE of the alloy. Nonetheless, the alloy must be essentially free of these elements (less than 0.02 percent carbon and 0.0l percent boron should be present) because their presence re-sults in the formation of carbide and boride precipitates which are incoherent and embrittle the alloy.
Although gold has a favorable effect on the SFE of the alloy and may be present, it cannot be used in lieu of tantalum and niobium since its low melting point does not allow it to serve as a nucleating agent.
The solubility of zirconium in cobalt-nickel alloys is too limited for it to be effective in raising the SFE of the alloy. Manganese presents alloying difficulties and copper has a low melting point and does not act as a nuclea-ting agent. Copper also lowers the corrosion resistance of ~ ~5~8 ~
the alloy. Aluminum has a low melting poin~ and does not act as a nucleating agent and the presence of aluminum in nickel containing alloys results in the formation o-f Ni3Al intermetallic compounds which raises in strength and hard-; ness o-f the alloy and lowers its duc~ility. Titanium acts in a similar fashion by causing the ~ormation o-f Ni3Ti. Tin may be incorpora~ed in the alloy due to its favorable effect on SFE of the alloy as well as its ability to lower its melt-ing point.
Iron ~hich also has a favorable effect on the SFE of a cobalt-containing alloy but its presence in significant amounts lowers the flowability and the corrosion resistance of the alloy and iron does not act as a nucleating agent.
Ruthenium, osmium, silicon, molybdenum, tungsten, iridium and platinum have unfavorable effects upon the SFE
of cobalt containing alloys and encourage the formation of the less ductile HCP phase and should not be present in significant amounts. Less than 1 percent silicon and tung-sten should be present and molybdenum should not be present in quantities of more than 2 percent. Sulphur content should not exceed 0.02 percent.
~ rom the tables below, it is apparent that the basic alloy of this invention has properties which exceed those of Type III dental gold. Accordingly, the proportions of cobalt, nickel, and chromium in the alloy can be present outside the specific ratios indicated above and still obtain an alloy having mechanical properties essentially equivalent to Type III gold dental alloys. This is possible because some HCP crystal structure can be tolerated and yet the alloy will exhibit properties -for a cro~n and bridge alloy . -'~ ~ ' ` ' '' , :

~ 5~7~8of the same level as Type III dental gold alloys. Accord-ingly, an alloy which is essentially composed, by weight, of about 10-60 parts cobalt, 17-2~ parts chromium, 20-75 parts nickel and up to 3 percent niobium or 6 percen~tantalum made according to this invention will provide a dental alloy suited for crown and bridge applications. In the pre~erred alloy, 50 parts nickel, 30 parts cobalt, and 20 parts chro-mium form the alloy base to which is added 2-6 percent tan-talum or 1-3 percent niobium. The presence of other elements can be tolerated only to the extent that they do not cause significant deterioration in the necessary mechanical pro-perties of the alloy.
The chromium content should not be materially reduced or increased beyond the a~orementioned ranges. An alloy hav-ing a low chromium content highly corrodes while excessi~e chromium content causes embrittlement. Increased cobalt content increases the quantity o~ the undesirable HCP struc-ture~ whereas increased nickel content increases the quantity of the desirable FCC structure in the alloy. The alloy of this invention, however, provides an excellent balance for crystal structure and mechanical properties.
The examples next set forth illustrate the proper-ties o-~ the alloy and the criticality o~ carbon and molybdenum.
All alloys were made under a non-oxidizing atmosphere using induction type heating units to avoid carbon contamination.
The alloys were cast by normal dental techniques in phos-phate-bonded investments at 1600F under a non-oxidizing atmosphere and quenched in cold water.
Table I describes compositions wherein the amounts of critical elements especially molybdenum and carbon in ~ 7~
Examples 1 and 2 are outside the essential rangGs claimed.
The physical properties of these alloys summarized in Table II are the ranges of four specimens made from each composi-tion. The values of the yield strength and ductility show that alloys 1 and 2 ~ail to meet the essential criteria for alloys for crown and bridge applications in one or more respects, i.e., low elongation, and/or undesirable hardness by virtue of high yield strength.
Example 3 is Type III gold alloy having the currently acceptable properties required for crown and bridge applica-tions.
Examples 4-8 are alloys prepared such that their cobalt-nickel-chromium ratios are balanced to procluce an alloy of high SFE that crystallizes in the FCC structure.
Examples 9-12 are alloys prepared such that their cobalt-nickel-chromium ratios are not desirable since their ratios encourage the alloy to crystallize in the less ductile HCP structureO
Examples 13-22 are of the same compositions as alloys 4-8 but contained tantalum or niobium as an additional element to further raise the alloys SFE as well as to act as a nu-cleating agent.
Examples 23-28 are of the composition as alloys 9-12 but contained tantalum or niobium to raise the alloys SFE and decrease the concentration o~ the HCP phase as well as to act as a nucleating agent. Although these alloys possess mechanical properties that make them suitable -for use in lieu o~ Type IV dental gold alloy for long span bridges which require a higher yield strength of 37,000 to 49 9 000 and a ductility of 4 percent or more, the alloys are not suitable '.

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for the processing of crowns or regular bridges.
Examples 29-32 represent the extreme concentrations of the cobalt-nickel-chromium alloys to which the extreme concentration of tantalum or niobium is added. These alloys may be used for long span bridges but not for crowns or regular bridges.

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~ 50788 TABLE I
ALLOY COMPOSITIONS - WEIG~ PERCENT
Example Ni Co Cr Nb Ta Mo C OT~ERS
1 14.2 52 26.1 -- -- 4 0.22 Fe 1.2, Si 0.58 Mh 0.7 `
2 2.7 61.53 27.66 -- -- 4.27 0.22 Fe 1.27, Si 0.57 M~ 0.66, W 1.05

3 GOLD TYPE III ALLOY

4 70 10 20 :. . .

13 69.3 9.9 19.8 1 -- --1~ 68.6 9.8 19.6 -- 2 --66.3 12.9 19.8 1 -- --~0 16 65.7 12.7 19.6 -- 2 17 59.4 19.8 19.8 1 -- --18 58.8 19.6 19.6 -- 2 --l9 49.5 29.7 19.8 1 -- --49.0 29.4 19.6 -- 2 --21 44.5 34.6 19.8 1 -- --22 44.1 34.3 19.6 -- 2 --23 39.2 39.2 19.6 2 --24 3~.4 38.4 19.2 -- 4 2S 29.4 49 19.6 2 --26 28.8 48 19.2 -- 4 71~!3 TABLE I-cont'd ALLOY COMPOSITIONS-WEIG~ PERC:~Nr Example Ni Co Cr Nb Ta C Others : 27 19.6 58.8 19.6 2 --28 19.2 57.6 19.~ -- 4 29 48.5 29.1 19.4 3 --47.0 28.3 18.7 -- 6 31 65.0 12.6 lg.4 3 --32 63.0 12.2 18.8 -- 6 , . -' `:

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TABLE II
~ECHANICAL P~OPERTIES
Example Yield StrengthDuctility UTS Maximum X 103 psi Percent X 103 psi Casti~ility Gauge 1 60.0 10 97.5 24 2 70.0 5.5 85.0 2 3 32.0 23 56.0 28 4 28.2-35.4 20 -29 57 -64.1 24 23 -28.7 27 -35.7 51 -59.4 2~
6 22.1-28.2 28 -37.8 50 -58.5 24 7 22.4-29.1 28.7-36.7 51 -57.9 24 8 26.2-34.9 22.9-34.5 48 -57.1 24 9 25.1-37.8 19 -29.9 49.1-54.9 24 23.4-30.9 19 -39.6 47 -56.2 24 11 22 -31.3 14 -23.5 ~6 -57.9 24 12 1~.3-30.1 4.9-10.1 40 -51.1 24 13 31 -33.3 26 -27.6 59.1-61.9 28 14 29 -30.5 28 -29 59 -60.3 28 27.9-29 30 -32 57 -58.9 28 16 28.3-31.1 31 -32.7 58 -59.~ 28 17 27 -29.2 32 -34.1 57.1-59.3 28 18 27.7-29.3 33 -35.2 57.4-59.7 28 19 25 -27.3 33 -36.3 52.5-54.9 28 25 -26 34 -37.1 51 -53.2 28 21 2~.9-32.7 23.1-25.3 48.7-51.1 28 22 30.1-31OS 25.2-27.1 49.2-51.4 28 23 38.3-~2.3 17 -18.5 60 -61 28 24 36 -37.1 25 -27.2 61.4-6~ 28 39.7-42.3 ~7.1-19.3 62 -6~.5 28 26 41.3-~3.7 18.2-18.9 61.5-63.4 28 -19- ~

~ ~ 5~ ~ 8 T~BLE II-cont'd ME~ICAL PROPERTIES
ExampleYield Strength DuctilityUTS Maximum X 103 psi PercentX 103 psi Castibility Gauge 27 46.8-49.6 12.9-14.16~3.7-71.1 28 28 45.7-47.6 14 -16.369.1-7~.7 28 29 46.0-48.3 l9 -20.369.6-73.4 28 42.1-44.2 19 -20.769.4-72.4 28 31 41.0-43.3 15.2-16.263.2-67.6 28 32 42.~-46.6 22.5-24.867.2-69.9 28 As will be apparent to persons skilled in the art, various modifications of the above described invention will become readily apparent without departure from the spirit and scope of the invention.

Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A highly ductile cobalt-chromium nickel dental alloy having a ductility of about 20 percent or more elongation, a yield strength of about 35,000 P.S.I or less and low work hardening character-istics suited for crown and bridge applications requiring deforma-tion by hand burnishing in the mouth of a patient, said alloy having no more than 0.02 percent carbon and being essentially free of boron, molybdenum, titanium, aluminum and tungsten to prevent the formation of hardening precipitates thereof in the alloy and having an alloy base consisting essentially of, by weight, about 10 to 60 percent cobalt, 17 to 24 percent chromium, and 20 to 75 percent nickel as the essential major alloying elements, and an element of the group consisting of tantalum and niobium alloyed therewith to promote a high rate of crystallization and provide uniformity and fineness of crystal size, said alloying element being present in an amount of up to about 4 percent tantalum or an equivalent atomic weight of niobium.

2. An alloy according to claim 1 wherein nickel is present, by weight, in the alloy base in an amount about twice the amount of chromium plus an additional amount in the ratio to cobalt of between about 1 to 3 and 2 to 1.

3. An alloy according to claim 2 wherein the alloying element is present in the amount of 2 percent by weight tantalum or the equivalent atomic weight of niobium.

4. A dental casting made of the alloy of claim 1.

5. An alloy according to claim 2 wherein the alloy base consists essentially of about 20 percent chromium, 50 to 67 per-cent nickel, and 13 to 30 percent cobalt, by weight, and the alloying element is present in the amount of up to about 2 percent tantalum or the equivalent atomic weight of niobium.

6. A dental casting made of the alloy of claim 5.

7. A dental casting made of the alloy of claim 2.

8. The method of Making a cobalt-nickel-chromium alloy suited for use as a gold alloy substitute for dental crown and bridge applications requiring hand burnishing in the mouth of a patient and having a controlled high stacking fault energy, a ductility of about 20 percent or more elongation, a yield strength of about 35,000 psi or less, and low work hardening characteristics, said alloy having an alloy base consisting essentially of about 20 to 75 percent nickel 17 to 24 percent chromium, and 10 to 60 per-cent cobalt, by weight, comprising the steps of providing nickel in an amount of about twice as much as the chromium present, of providing the remainder of the alloy base by adding cobalt and an additional amount of nickel in the ratio of nickel to cobalt of between about 1:3 and 2:1, by weight and the step of stabilizing the uniformity and fineness of the crystal structure of the alloy by adding an element selected from the group consisting of nio-bium and tantalum in an amount of up to about 4 percent tantalum, by weight, or the equivalent atomic weight of niobium to promote a high rate of crystallization and provide fineness and uniformity of grain size in the alloy.